Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session H16: Focus Session: Tip-enhanced Nano-optics and Spectroscopy of Quantum Dots |
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Sponsoring Units: DMP Chair: Khaled Karrai, Ludwig-Maximilians University Room: LACC 404A |
Tuesday, March 22, 2005 8:00AM - 8:12AM |
H16.00001: Tip-enhanced spectroscopy from ordered silver nanoparticle arrays Erin Orazem, Dave Bussian, Sarah Cross, Jason Schmidt, Melissa Summers, Steve Buratto I will report on recent efforts from our group to develop new methods for enhanced spectroscopy from metal nanoparticle arrays. Triangular-shaped silver nanoparticles (20 -- 200 nm) fabricated via nanosphere lithography have shown dramatic enhancement (10$^{6}$ -- 10$^{10})$ of the resonance Raman spectrum of R6G, which was attributed to the shape of the Ag triangle.\footnote{ J. Schmidt, S. Cross, S. Buratto J. Chem. Phys., \textbf{121}, 10657 (2004).} In addition to the large surface-enhanced resonance Raman spectroscopy (SERRS) enhancement we have also observed enhanced luminescence from the Ag nanoparticle films. In order to further understand the shape dependence of the electric-field enhancement we have used near-field scanning optical microscopy (NSOM) to map both the SERRS and luminescence for a single 200nm Ag nanotriangle. The enahancement is largest for the apex of the triangle or near sharp points. In addition, I will discuss the progress in tip-enhanced SERS (TE-SERS) from these same nanoparticle arrays. In TE-SERS the electric field enhancement is due to the large coupling between the metal tip and the Ag nanoparticle and can be mapped and quantified. [Preview Abstract] |
Tuesday, March 22, 2005 8:12AM - 8:24AM |
H16.00002: Electromagnetic response in Scattering-type Scanning Near-field Optical Microscopy Javier Aizpurua, F. Javier Garcia de Abajo, Pedro Miguel Echenique The ability to perform scattering-type Scanning Near-field Optical Microscopy (s-SNOM) and spectroscopy has been demonstrated in different ranges of the spectral response. The information obtained from a particular sample is based on the near-field interaction driven at the s-SNOM cavity. The incoming light activates plasmonic and phononic resonances, which are extremely dependent on the conditions of the electromagnetic cavity. We present a theoretical model of the s-SNOM cavity and perform exact electromagnetic calculations of the optical response of this nanosystem with use of a boundary element method in the frequency domain. We study the coupling between tip and sample, and the influence of the nano and micro-environment in the optical response of this nanosystem both for metallic and phononic materials with resonances in the visible and phonon region of the spectrum respectively. We focus on the study of the response of SiC terraces as a structure with potential application in phononic devices. The results obtained here can be used to interpret and to generate input for optimal performance of the s-SNOM. [Preview Abstract] |
Tuesday, March 22, 2005 8:24AM - 8:36AM |
H16.00003: Microscopic near-field optics of metallic nanoparticles Yongqiang Xue Microscopic near-field optics of metallic nanoparticles: The further miniaturization of integrated optical devices requires investigating optical elements with dimensions on the nano-scale. Methods are therefore needed for detecting and guiding light on a scale much smaller than the wavelength of the light. Near-field optics, which exploits evanescent rather than propagating light fields, attracts a lot of attention as a promising way of circumventing the diffraction limit. Since the evanescent fields bound to the nanostructures decay in intensity within a fraction of the light wavelength, they carry information about sample features on a sub-wavelength. It is clear that to investigate light-matter interaction in a spatial extension much less than the optical wavelength, one can not in general have confidence in the macroscopic electrodynamics so far popular in near-field optics and photonic band-structures. Instead a microscopic approach treating rigorously the local-field effect is highly desirable. In this work we present a microscopic theory of near-field optical effect in single and coupled metallic nanoparticles. Our theory is based on the Lagrangian formulation of semiclassical electrodynamics, where we treat the nanoparticle optical response quantum-mechanically within the jellium model. We discuss the insights obtained from such microscopic analysis for understanding light confinement in subwavelength structures and near-field mediated electromagnetic energy transport through resonant excitation of surface plasmon polariton modes. We also discuss light scattering by charged metal nanoparticles. [Preview Abstract] |
Tuesday, March 22, 2005 8:36AM - 9:12AM |
H16.00004: Tip-confined near fields for locally probing optical constants and fluorescence Invited Speaker: Nanoscopic light focusing occurs near nanoscale optical/infrared antenna structures (eg small particles or AFM tips), through dielectric and geometric physical mechanisms. Such highly confined light probes linear optical/infrared properties at $<$20 nm spatial resolution, in two realizations. (i) Scattering-type SNOM employing tapping AFM, free-space illumination, and heterodyne detection. This works well with even long-wavelength infrared. Maps of topography and of complex light scattering efficiency are simultaneously recorded. The latter can determine the local complex optical index or dielectric function as has been shown, by electrostatic modelling, with free electrons in doped semiconductors, phonon resonance in polar crystals, nanocomposite polymers, and proteins. (ii) Tip-on-aperture SNOM employing fiber-guided illumination through an aperture on which a metal tip is grown. Tip-confined fluorescence imaging of single, DNA-tagging fluorophores has been achieved together with high-resolution topography, at negligible interference from background illumination or photobleaching. (i) Keilmann{\&}Hillenbrand, Phil. Trans. R. Soc. Lond. A 362, 787 (2004); (ii) Frey{\&}Guckenberger, Phys. Rev. Lett., in print. [Preview Abstract] |
Tuesday, March 22, 2005 9:12AM - 9:24AM |
H16.00005: Size dependence of the exciton g-factor in self-assembled CdTe/ZnTe quantum dots Thang B. Hoang, Sebastian Mackowski, Howard E. Jackson, Leigh M. Smith, Piotr Wojnar, Grzegorz Karczewski, Jacek Kossut We study exciton spin splitting of single quantum dots in a highly inhomogeneous self-assembled CdTe/ZnTe quantum dot sample. By spatial imaging of individual dots in applied magnetic field (using a solid immersion lens) and by analyzing the circular polarization of the PL emissions, we measure both the spin splitting and the diamagnetic shift of zero-dimensional excitons. We determine that the absolute value of exciton g-factor decreases from 4 to about 2 as the emission energy decreases from 2.35 eV to 1.85 eV. At the same time, we observe an increase of the diamagnetic shift up to 100 $\mu $eV. Such small values indicate strong spatial confinement of the excitons, as the diamagnetic shift corresponding to the bulk Bohr radius ($\sim $10nm) which equal 600 $\mu $eV. From the diamagnetic shift we estimate that the diameter of CdTe quantum dots ranges from 1nm to 6nm. These results imply that the g-factor and thus the spin splitting of a quantum dot exciton is strongly enhanced for quantum dots with smaller size of the confining potentials. The work was supported by NSF grants nr 9975655 and 0071797 and PBZ-KBN-044/P03/2001. [Preview Abstract] |
Tuesday, March 22, 2005 9:24AM - 9:36AM |
H16.00006: Exchange coupling between magnetic and non-magnetic quantum dots Leigh M. Smith, Sebastian Mackowski, Tak Gurung, Aziz M. Majidi, Mark Jarrell, Howard E. Jackson, Fedir V. Kyrychenko, Chris J. Stanton, Grzegorz Karczewski We study the exchange interaction between non-magnetic CdTe and magnetic CdMnTe quantum dots (QDs) grown epitaxially in the same layer by means of single dot magneto-photoluminescence spectroscopy. Very narrow emission lines ($\sim $0.1 meV) exhibiting negative exciton g-factors are attributed to non-magnetic QDs. On the other hand, much broader PL lines ($\sim $3-4meV), which for circularly polarized resonant excitation at B=0 T feature strong polarization of the emission confirm the presence of magnetic CdMnTe QDs. We demonstrate that in applied magnetic field the exciton Zeeman splitting in a single CdTe QD is larger for resonant $\sigma ^{-}$ (against the field) excitation than for $\sigma ^{+}$ (along the field) excitation. We interpret these results as an indication of the exchange interaction between excitons in the CdTe QD and ferromagnetically aligned magnetic inos confined to neighboring CdMnTe QDs. Remarkably, in spite of the external field, the spin alignment of Mn ions in QDs can still be controlled by the polarization of the excitation. Thus, the excitons in non-magnetic QDs provide an extremely sensitive probe of the light-induced magnetization. The work was supported by NSF grants 9975655, 0071797, 0325474, and 0312680 and PBZ-KBN-044/P03/2001. [Preview Abstract] |
Tuesday, March 22, 2005 9:36AM - 9:48AM |
H16.00007: Temperature dependence Infrared and Raman studies of III-V/II-VI core-shell nanostructures Felicia S. Manciu, Bruce D. McCombe, Bernard A. Weinstein, Robert E. Tallman, Derrick Lucey, Y. Sahoo, Paras N. Prasad The temperature dependence (8 K $<$ T $<$ 300 K) of optical phonon modes confined in InP/II-VI core-shell nanostructures have been investigated by far-infrared (FIR) and Raman scattering spectroscopies. The core-shell nanostructures were fabricated by colloidal chemistry and characterized by transmission electron microscopy and X-ray diffraction prior to being embedded in a polycrystalline CsI matrix for the present studies. The FIR measurements of InP/ZnSe sample exhibits three absorption features, one clearly due to the Froelich mode of the InP cores, and the others related to modes associated with the shell layer and its coupling to the matrix. Strong mixing of the characteristic vibrations of each constituent material was observed for InP/ZnS sample. Raman scattering (457.9 nm excitation) features were determined without polarization selection in the backscattering geometry. Interesting T-dependent resonant Raman effect of the surface optical phonon modes has been discovered in InP/ZnSe sample. Reasonable agreement is obtained between the Raman and FIR results, as well as with theoretical calculations. [Preview Abstract] |
Tuesday, March 22, 2005 9:48AM - 10:00AM |
H16.00008: Tuning the spin properties of excitons in CdTe/ZnTe self-assembled quantum dots Kapila P. Hewaparakrama, Sebastian Mackowski, Howard E. Jackson, Leigh M. Smith, Grzegorz Karczewski, Jacek Kossut, Wolfgang Heiss With annealing, CdTe/ZnTe self-assembled QDs become larger through Zn and Cd interdiffusion at the dot-barrier interface. Here we determine the exchange splitting, diamagnetic shift and exciton g-factor through imaging hundreds of single CdTe QDs in the as-grown and annealed QD samples using a solid immersion lens and slit-confocal microscopy. An analysis of the polarized QD emission in external magnetic field demonstrates that annealing strongly affects the spin properties of the QD excitons. First, we find that the statistical distribution of the exchange splitting at B=0 T is more than two times narrower for the annealed QD sample. In applied magnetic field we observe a reduction of the average exciton g-factor upon annealing, which is accompanied with a substantial increase of the diamagnetic shift. These results show a way to tune the spin properties of excitons in self-assembled QDs. The work was supported by NSF grants nr 9975655 and 0071797 and PBZ-KBN-044/P03/2001. [Preview Abstract] |
Tuesday, March 22, 2005 10:00AM - 10:12AM |
H16.00009: Probing excited states in self-assembled quantum dots with resonant Raman scattering Amensisa Abdi, Thang B. Hoang, Sebastian Mackowski, Leigh M. Smith, Howard E. Jackson, Jan Yarrison-Rice, Jacek Kossut, Grzegorz Karczewski We investigate the distribution of excited states in as-grown and annealed CdTe self-assembled quantum dots grown by molecular beam epitaxy using room temperature resonant Raman scattering (RRS). We observe that the Raman intensity of the first LO ZnTe phonon at 202 cm$^{-1}$ is strongly dependent on the laser excitation energy. For the as-grown sample, the RRS intensity peaks at an energy of 2.08 eV with a width of 90 meV. For the annealed sample, the RRS intensity peaks at a higher energy, 2.11 eV, with a narrower width of 70 meV. The widths reflect the distribution of excited state energies. The peak positions indicate an energy separation between excited and ground states of $\sim $80 meV and $\sim $60 meV for the as-grown and annealed CdTe quantum dots respectively. The smaller separation for the annealed dots suggests that interdiffusion results in a weaker confining potential. We acknowledge the support of NSF DMR 0071797 (U.S.) and PBZ-KBN-044/P03/2001 (Poland). [Preview Abstract] |
Tuesday, March 22, 2005 10:12AM - 10:24AM |
H16.00010: Electric field modulated optical saturation absorption of CdSe quantum dots Bin Shi, Zuoming Zhao, Pak-Sam Chang, Ya-Hong Xie, Chander Radhakrishnan, Harold Monbouquette We study the nonlinear optical absorption of CdSe quantum dots in different external electrical. The absorption of CdSe quantum dots clearly shows saturation absorption behavior with incident light wavelength shorter than the absorption edge of CdSe quantum dots, while the saturation intensity changes with external electrical field. The origin of such a decrease of saturation intensity with increased external field is discussed. The application of the dependence of saturation absorption on electrical field is also discussed. [Preview Abstract] |
Tuesday, March 22, 2005 10:24AM - 10:36AM |
H16.00011: Modulated electroreflectance from multilayer InGaAs quantum wires and dot chains Zhixun Ma, Todd Holden, Zhiming Wang, Lyudmila Malikova, Gregory Salamo In coherent multilayer quantum wire structures, the strains are accommodated elastically without plastic relaxation, resulting in a modification of the electronic states even in the barriers. The electronic properties of strained InGaAs/GaAs quantum wire/chain systems have been investigated using modulated electroreflectance and theoretical calculation based on an envelope function method. Various optical transitions such as the state splitting of heavy-light holes were observed and assigned to be transitions from InGaAs quantum wires/chains and two wetting layers with different thickness. A very important finding is that the strain-induced heavy-light hole splitting not only happens in the InGaAs wells, but also in GaAs barriers. These experimental results uncover the nature of the residual strain effects on the energy structure and confirm the theoretical prediction. [Preview Abstract] |
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